In a conventional image exposure device, as used e.g. in an electrophotographic printer or in an imagesetter, an image is formed on a radiation-sensitive body by exposing the body image-wise by a laser beam from a laser. In a conventional electrophotographic printer, the image on the radiation-sensitive body is a latent image, which is developed into a visible toner image, that is subsequently transferred to an image carrier such as a sheet of paper. In a conventional imagesetter, the radiation-sensitive body usually is the image carrier itself, such as a roll of photographic film; on the film, a latent image is exposed, that is afterwards developed in a processor into a visible image.
The laser in the image exposure device may be a semiconductor laser; the laser beam is modulated based on an image signal that corresponds to the image.
A semiconductor laser has a threshold current level I.sub.th (expressed e.g. in mA). A laser beam is emitted if the electrical driving current I applied to the laser is larger than the threshold current level I.sub.th. FIG. 1a shows two curves, one for a low temperature and one for a high temperature, with corresponding threshold current levels I.sub.thLOW and I.sub.thHIGH ; the temperature dependence of I.sub.th is discussed below. Below the threshold current level I.sub.th, the radiation intensity RI (expressed e.g. in mW) of the radiation emitted by the laser is very small, and in fact, the emitted radiation is not coherent, i.e. it is not really laser radiation. Thus, in general only the portions of the curves for I larger than I.sub.th are useful for imaging purposes; therefore the portions of the curves in FIG. 1a for I smaller than I.sub.th are drawn as dashed lines. The driving current I that is applied to the laser is modulated based on the image signal, so that the intensity RI of the emitted laser beam is a function of the image signal.
The laser threshold current level I.sub.th is not constant, but depends on different factors, such as the ambient temperature, the ageing of the laser, differences between individual lasers. Fluctuations in the threshold current level cause significant variations of the laser beam intensity; as shown in FIG. 1a, the threshold current level changes from I.sub.thLOW to I.sub.thHIGH when temperature changes from low to high.
In order to obtain good image quality, the intensity RI of the laser beam should depend only on the image signal and should not change with the factors influencing the threshold current level I.sub.th. Therefore, it is customary to control the laser, usually by means of a feedback circuit as known in system control theory. Customarily the required intensity of the laser beam is determined and a signal based on the measured intensity is fed back to the driving circuit of the laser in order to control the laser driving current. In this way, the effect of disturbances, influencing e.g. the laser threshold current level, can be reduced.
Two known methods to control the laser are illustrated by means of FIG. 1a. Suppose that, at low temperature, an image or a portion of an image must be written at the laser beam intensity corresponding to point B in FIG. 1a. In a first method the laser is driven from point O to point B in one step; in a second method the laser is driven from O to B via A in three steps. Both methods are now discussed in detail.
In the first method, at the moment the image must be written, the laser starts from zero driving current and zero radiation intensity in point O, and the laser driving current is increased until the driving current corresponding to point B is reached. A disadvantage of this first method is that it takes a long time for the driving current to reach point B, since the operating point of the laser must pass the whole way from O via A to B (see FIG. 1a). Because of the long time that is required, the imaging speed is limited.
In the second method, the laser is driven from O to B via A in three steps. In the first step, the laser is driven from O to A. Then, in the second step, the laser is kept at point A, until an image, or more precisely, until a non-zero portion of an image has to be written. Finally, in the third step, when a non-zero portion of the image must be written, the laser is driven from A to B. Essential in this method is that, beforehand, the driving current is set to an initial driving current that equals the threshold current level I.sub.th Low before the image is written. The advantage is that, when an image is to be written, the laser reacts promptly to the change in laser driving current corresponding to a change from point A to point B, thus overcoming the problem of the limited imaging speed of the first method. A disadvantage of this second method is that the laser is kept for some time at operating point A, which means that some radiation is being emitted before the actual image is written. This means that the radiation-sensitive body is exposed by the radiation emitted in A. Therefore, specific countermeasures may have to be taken; in order to avoid that an image is formed by the radiation emitted in A, in an electrophotographic printer for example the electrophotographic parameters such as the cleaning potential may be adjusted (the cleaning potential is discussed e.g. in EP-A-0 788 273). However, these countermeasures have drawbacks (adjusting the cleaning potential, for example, may result in increased carrier loss).
Variants of the second method exist: instead of setting the driving current to an initial value that equals the threshold current, the driving current may be set to a value about the threshold current value, or, alternatively, the driving current may be set to a value below the threshold current value.
Thus, the problems of the first and the second method discussed above are either the limited imaging speed, or undesired emitted radiation, or both.
Patent U.S. Pat. No. 5,416,504 discloses an image exposure device that implements another variant of the second method discussed above (the second method involves driving the laser to B in FIG. 1a in three steps: driving the laser from O to A; keeping the laser in A; driving the laser from A to B). In this device, the initial driving current is set about the threshold current level of the semiconductor laser (i.e. the laser is driven from O to about point A in FIG. 1a) in two substeps: first the driving current is set to a first value near the threshold current level, using the output of a counter circuit, and then the driving current is adjusted to a second value about the threshold current level by incrementing or decrementing this counter. The counter is incremented or decremented depending on the difference between a reference voltage and a monitor voltage that indicates the actual laser beam intensity and that is obtained as follows: the light from the laser is input to a monitor diode; the monitor diode outputs a current that is converted to the monitor voltage. When the monitor voltage equals the reference voltage, this indicates that the laser beam has reached a predetermined intensity. Finally, in order to write an image, the input image signal is added to the counter output value (the counter output value corresponding to the above second value of the driving current) and the sum is converted into an electrical current for driving the laser. The counter output can only change gradually; the purpose hereof is to stabilise the control of the driving current. The laser beam is monitored only during the time that no image signal is applied to the laser; there is no feedback during exposure of the image. A disadvantage of this device is, as mentioned above, that undesired radiation is emitted which exposes the radiation-sensitive body. Another disadvantage is the high complexity of the circuitry that controls the laser driving current. Yet another disadvantage is that there is no feedback, and hence disturbances--influencing e.g. the laser threshold current level--are not coped with during image exposure.